Yemeni officials are blaming al-Qaida for a bomb that destroyed secret police headquarters in the southern port city of Aden Saturday.
No casualties are reported. Al-Qaida fighters are said to have moved into the city after pro-government forces took it back from Houthi rebels several weeks ago.
The al-Qaida takeover of parts of Aden is seen as another troubling development in a five-month-old civil war that has already killed thousands.
The United States considers al-Qaida's Yemen affiliate to be the terrorist group's most dangerous branch. The U.S. has launched dozens of drone strikes against al-Qaida members in Yemen, killing a number of senior leaders.
Al-Qaida has taken advantage of the chaos on the ground in Yemen to grab several strategically important areas including the capital of eastern Hadramawt province. But the Sunni group has generally kept a low profile in the fight against the Shi'ite Houthi rebels.
The Iranian-backed Houthis seized the Yemeni capital of Sana'a almost a year ago, forcing President Abdu Rabu Mansour Hadi to flee to exile in Saudi Arabia.
The Saudis began airstrikes on the Houthis in March and have sent heavy weapons to pro-government fighters on the ground, allowing them to make some progress against the Houthis.
But the airstrikes and ground combat have created a humanitarian calamity in Yemen. The United Nations says about 4,500 people have been killed and 80 percent of Yemeni civilians desperately need food, medicine, and other humanitarian aid.
U.N. humanitarian chief Stephen O'Brien calls the scale of suffering in Yemen "incomprehensible" and warns that unless the fighting ends soon, "there will be nothing left to fight for."
U.N. efforts to broker a peace deal in Yemen have so far failed.